Quantitative analysis in budgeting (high-low method)

Hi sir I watched the lecture related to this topic and while you did the example question and got the total cost equation as TC= 32500 + 75x. This equation gives the total cost for the highest and lowest level of activity. However, when I use this equation to find the total cost of another activity level, say for example 400 units, the answer I get does not match to the cost given in the question For 400 units, in the question, the cost is 65,000. But when I insert this into the equation, 32500+(400x75) = 62,500. Can you please explain? Thanks in advance